Thermionic modulating system



March 13, 1934. N. WELLS 1,950,464

THERMIONIC MODULATING SYSTEM Filed Nov. l0. 1930 z/M/WL/ ATTORNEY Patented Mar. 13, 1934 UNITED STATES PATENT FFIE Norman Wells, London, England, assigner to Radio Corporation of America, a corporation of Delaware Application November 10, 1930, Serial No. 484,514 In Great Britain November 16, 1929 4 Claims.

This invention relates to thermionic tone modulating systems suitable for use in wireless signalling. More particularly the invention relates to so-called absorber modulating systems, i. e.,

l modulating systems of the kind in which the modulation voltages are applied to the input circuit of a thermionic valve which is rendered more or less conductive thereby, and which accordingly takes more or less current through an anode resistance fed from a power source, the consequent varying voltage drop in said anode resistance serving to vary the anode potential applied to a Valve in whose circuits the modulation is to be eifected.

The invention has for its objects to increase the energy per impulse over each period of the modulation frequency and thereby to give clearer signals.

According to this invention, the anode circuit of an absorber valve in an absorber modulating system is shunted by a capacity.

The invention is illustrated in the single gure in the accompanying drawing.

Referring to the figure which shows one form of construction in accordance with this invention, the output terminals of a no-te frequency oscillator 1 are connected one to earth E and the other through a coupling condenser 2 to the marking contact 3 of a keying relay 4. The said marking contact is connected through two resistances 5, 6, in series to the tongue '7 of the relay which tongue is connected to the grid 8 of a triode 9 acting as an absorber valve. The junction point 10 of the two resistances is connected through a negative bias battery 11 of suitable voltage (for example 180 volts) to earth.

The anode l2 of the absorber valve 9 is connected through a resistance 13 (for example, 50,000 ohms to 100,000 ohms) to the positive terminal of a source of anode potential (not shown) at HT, said anode 12 being also connected through a condenser 14, for example, of .05 microfarad or .1 microfarad to earth. The valve 9 is coupled to the grid 15 of a following valve 16 through a suitable negative bias battery 17 Whose positive terminal is connected to the anode 12 of the valve 9 and whose negative terminal is connected to the grid 15 of tube 16. The valve 16 also acts as an absorber, i. e., it is in effect a magnifier of the absorption due to the valve 9. The valve 9 may thus be regarded as a sub-absorber and the following valve 16 as a main abso-rber. The anode 18 of the main absorber valve 16 is connected through suitable resistances 19 to the positive terminal of the source of anode potential connected at HT. and also to the anode 20 of a (Cl. Z-19) transmitter valve 22 in the amplifier stage 23 in whose circuit modulation is to be effected. The amplifier stage 23 may include a source of carrier frequency oscillations or may be driven by a source of carrier frequency oscillations not shown. When the key 7 is operated the carrier wave is modulated at the keying frequency by the tone frequency from i. It Will be seen that with this arrangement when the main absorber valve 16 takes current the voltage drop across resist- 05l ances 19 will cause the reduction of the anode potential applied to the anode 20 of the transmitter amplifier Valve 22. The valve 22 may be the final stage in a transmitter but preferably this stage, including the tube 22, drives a power amplifier stage 24 which is also energized by the source connected at HT.

It has been found that the provision of the condenser 14 between the anode of the sub-absorber valve and earth results in an increase in the energy per impulse over each period of the modulation frequency as compared to arrangements in which the anode of an absorber valve is not capacity shunted. The increase in energy is very considerable and in one case the provision of the condenser has resulted in a transmitter which when adjusted to its best performance caused energy to leave the associated aerial during only 25% of the marking period to increase the energy so that it left the aerial during approximately 95% of the marking period. For a given effect the value of the capacity depends upon the frequency of modulation and upon the value of the associated anode resistance, and is preferably chosen togive maximum power output Without reducing the percentage of modulation below In cases where it is desirable to retain a fair percentage of un-modulated carrier Wave, such as when very high telegraphic speeds are called for 95 and the conditions are suitable, the size of the capacity may be increased. In fact the percentage of modulation may be definitely controlled according as the value of the capacity is increased beyond the normal.

In the keying system illustrated in the figure, the values of the resistances 5 and 6 may be so chosen that sparlnng is eliminated at the keying contacts; thus if resistance 6 be made 200,000 ohms and resistance 5, 5,000 ohms, the effective 1.05 shunt load on the tone modulator will differ very little Whether the contacts be open or closed, since the greater portion of the alternating current ows through resistance 5 in either case and the contacts serve mainly to transmit only 110 alternating current potential to the grid of the valve.

The invention is of substantial advantage in connection with short wave transmitters, such, for example, as those employed at the present time in the well known beam system.

Having now particularly described and ascertained the nature of my said invention and in what manner the same is to be performed, I declare that what I claim is:-

1. An absorber modulating system comprising, a source of modulating potential, an absorber valve, keying means interposed between said source and said valve for rendering the valve conductive a-nd non-conductive comprising, a key, a pair of resistances of diierent values connected in series between the terminals of said key, a source of potential for applying bias to the junction point of said resistances to render the valve conducting and non-conducting under the control of the key, said resistances being of such value that sparking at the keying contact is substantially eliminated, a resistance in the anode circuit of said absorber valve, means for tapping off potential from said resistance for controlling a transmitter stage, and a condenser connected between the anode of said absorber valve and the cathode thereof.

2. In signalling means, a transmitter including, a thermionic amplifier valve adapted to amplify and translate high frequency energy, and means for controlling the operation of said valve comprising, an absorber valve having its anode connected by way of a resistance to a source oi' high potential and its cathode connected to ground, a connection between the anode of said absorber Valve and an electrode in said ampliier valve a second absorber valve having its anode connected by way of a resistance to said source of potential and its cathode connected to ground, a source of potential having its positive terminal connected to the anode of said last named valve and its negative yterminal connected to the control grid of said absorber valve, a key `including a movable element and a contact, a pair of resistances connected in series between said movable element and said Contact, a connection between said movable element and the control grid of said second absorber valve, a connection between the adjacent terminals or" said series resistances and ground, said connection including a potential source for applying negative potentials of different values to the control grid of said second absorber valve in dilerent positions of said key. and a capacity connected between the anode and cathode of said second absorber valve to increase the energy radiated by said transmitter during keying impulses.

3. A -thermionic repeater valve having a control electrode which determines the operativeness of said valve and means for keying said valve comprising, a second .thermionic valve having its anode connected through a high impedance and a source of potential to its cathode, a source of potential having its positive terminal connected to the anode of said second named valve and its negative terminal connected to the control grid of said rst named valve, a pair of resistances of different values, each having one terminal connected to the negative rterminal of a source of potential, the positive terminal of which is connected to ground, a xed contact connected to the free end of one of said resistanees, a movable contact connected to the free end of the other of said resistances, a connection between one of said contacts and the control grid of said second named valve whereby negative potentials of dierent value may be applied to the control grid of said second named valve, and means for increasing the keying action of said second named valve on said rst named valve comprising a capacity connecting the anode of said second named valve to the cathode of said second named valve.

4. Tone keying means comprising, a thermionic absorber valve having anode, cathode and control electrode, a source of potential connected through an impedance between the yanode and cathode of said absorber tube, a source of oscillations of tone frequency, a pair of resistances in series between 1e control grid of said absorber tube and said source of oscillations of tone frequency, said resistances being of different value, a source of potential connected between adjacent terminals of said resistances and ground, keying means comprising a Xed element and a movable element, said elements being connected in parallel with saidY series resistances, means for increasing the variation in energy intensity on the anode electrode of said absorber valve in different positions of said key comprising, a capacity connected between the anode and cathode of said valve, and a circuit for utilizing the potential drop through the resistance connected with the anode of said valve.

NORMAN WELLS. 

